1. Field of the Invention
The invention relates to an oscillator circuit comprising first and second transistors, whose emitters are intercoupled and jointly connected to a variable current source, the first transistor being connected in grounded-base connection, also comprising a parallel resonant circuit coupled to the collector of the first transistor, the collector being coupled to the base of the second transistor for a regenerative feedback of the circuit voltage.
2. Description of the Prior Art
Such an oscillator circuit is described in the published Netherlands Patent Application No. 147294.
When the prior art oscillator circuits are produced in larger quantities, the individual circuits may mutually show spread in the amplitude of the oscillator output signal, for example due to inaccuracies in the values of the circuit elements. In addition, the oscillator amplitude may vary due to inter alia temperature fluctuations or ageing.
The influence of such factors on the oscillator amplitude can be reduced by amplitude detection of the oscillator signal and by negative feedback of the d.c. voltage signal thus obtained to the variable current source. Such an automatic gain control (AGC) for the IF-signal in AM-receivers is known from, for example, the U.S. Pat. No. 3,284,713 and is not so suitable for integration because of the comparatively large capacitance required for amplitude detection. In addition the amplitude detection when used in an oscillator circuit introduces a certain degree of control inertia, which may result in unwanted transients.
The invention has for its object to provide an oscillator circuit comprising a simple stabilizing circuit, which can be easily integrated and stabilizes the oscillator amplitude at a predetermined value without any noticeable delay and highly independent of the spread in values of the circuit elements.
According to the invention, an oscillator circuit of the type described in the opening paragraph is characterized in that said collector is also coupled to a control input of the variable current source for an instantaneous variation of the current of the variable current source in dependence on the circuit voltage.
The invention is based on the recognition that stabilizing the oscillator amplitude is possible without amplitude detection by an instantaneous, negative feedback of the circuit voltage.
Such an instantaneous negative feedback is realized by applying the measure according to the invention. Then an increase or decrease, respectively of the emitter current produced by the variable current source instantaneously coincides with a decrease or increase, respectively in the conduction of the first transistor. If the oscillator amplitude or circuit voltage variation increases, for example as a result of the above-mentioned ambient factors, then also the emitter current variations increases, which for a suitably chosen transistor setting results in an instantaneous decrease of the collector current of the first transistor applied to the resonant circuit. The reverse holds for the case in which the oscillator amplitude decreases.
Although the collector current of the second transistor varies in the same phase as the oscillator signal this does not have any effect on the circuit voltage as the second transistor only functions as an emitter follower and its collector current flows to ground without passing through the resonant circuit.
A preferred embodiment of the oscillator circuit according to the invention is characterized in that the variable current source comprises a third transistor, whose collector is connected to the intercoupled emitters of the two first and second transistors and that the collector of the first transistor is coupled via an emitter follower to a first voltage divider, a first output of which is connected to the base of the second transistor and a second output to the base of the third transistor.
When this measure is applied, an adequate setting of on the one hand a sufficiently strong positive feedback to maintain an oscillation and on the other hand a sufficiently strong negative feedback for an acceptable stabilization of the oscillator amplitude can be obtained in a simple way.
A further preferred embodiment of such an oscillator circuit is characterized in that the current source comprises a fourth transistor, whose base is connected to a fixed base-biassing voltage and whose emitter is coupled to the emitter of the third transistor, both emitters being jointly connected to a constant current source, the current thereof at the biassing voltage of the third and fourth transistors flowing to a sufficient extent through the third transistor for amplifying the circuit voltage in the first transistor, such that regeneration of the circuit voltage occurs.
When this measure is applied, the emitter currents of the first and second transistors vary more than linear, with the circuit voltage which is fed back to the base of the third transistor, as a result of which the influence of the circuit quality on the amplitude of the circuit voltage decreases still further.
A simple base voltage bias of the first and fourth transistors is obtained in a still further preferred embodiment, in which a second voltage divider is arranged between a supply voltage and ground, a first output thereof being coupled to the base of the first transistor and a second output to the base of the fourth transistor.
The invention will now be described in greater detail by way of example with reference to the Figures in the accompanying drawing.